Hillary, the former U.S. secretary of state, has been a vocal opponent of the black market trade in wildlife parts, calling for a global strategy to protect wildlife in their environments and dry up demand for trafficked wildlife goods. (Related: "Blood Ivory" in National Geographicmagazine.)

"We strongly endorse a complete ban on ivory sales in the U.S. The global ban agreed [to] in 1989 was successful in stemming a previous killing spree. Over time, however, exceptions have eviscerated the international ban and illegal ivory is now routinely bought and sold under one or more loopholes, providing cover for illegal traffickers. These need to be closed and sanctions imposed on countries that continue to trade in ivory products."

For instance, "retailers need to stop selling ivory products. And businesses need to blow the whistle on government officials and institutions that have been corrupted by this lucrative, illegal trade." (Read more in National Geographic's A Voice for Elephants blog.)

"And as consumers, we should urge companies to help law enforcement authorities disrupt the transfer of tusks, rhino horn and wildlife products on ships, aircraft and trucks. Financial institutions should help to trace illegal transactions, freeze assets and impound ill-gotten gains from illegal trafficking."

There are small things people can do to help: The U.S. Postal Service is selling Save Vanishing Species stamps, which have raised over two and a half million dollars for species conservation.

"Only by working together can we beat this crisis, break the nexus between trafficking and terrorism, and make sure these incredible creatures will roam the earth for generations to come," the Clintons wrote.